In the 1960's and early 1970's, Robert Monroe of the Monroe Institute of Applied Sciences explored the effects of sound on the brain and discovered that he could produce a driving or entrainment of brain waves. Dr. Gerald Oster, a biophysicist, also investigating the effects of sound on the brain, discovered that pulsations called binaural beats occurred in the brain when tones of different frequencies were presented separately to each ear. The beat frequency equals the frequency difference between the two tones. Both Monroe and Oster began using electronic oscillators to provide tones with frequency, purity and intensity that can be precisely controlled.
U.S. Pat. No. 3,884,218 to Robert A. Monroe shows a method for inducing sleep by amplitude modulating a pleasing sound with a delta-rhythm signal that is referred to as an “EEG sleep signal.” The '218 patent uses sound to induce sleep by creating a specific signal that coaxes the brain into a relaxed state. This signal chosen in the '218 patent is chosen based upon its proximity to signals that are strong during normal sleep.
U.S. Pat. No. 4,191,175 to Nagle shows a method and apparatus for repetitively “producing a noise-like signal for inducing a hypnotic or anesthetic effect . . . ” by creating frequency bursts of digital pulses that are then passed through a pink noise filter to eliminate frequencies above a certain cut-off. The resultant signal is then passed through a band pass filter and used to drive an audible signal source.
An apparatus for electrophysiological stimulation is shown in U.S. Pat. No. 4,227,516 to Meland et al. in which a first signal above the delta-beta frequency range is modulated by signal within that range and applied to electrodes on the forehead of a user.
A learning-relaxation device of U.S. Pat. No. 4,315,502 has both lights for pulsing signals and sound means for a pulsing sound signal as well as a control means that can individually vary the light and sound signals.
U.S. Pat. No. 4,834,701 to Masaki shows a device similar to those used by Monroe and Oster with first and second generators with frequencies above 16 hertz and a frequency difference of 4 to 16 hertz sounded to lower the brain wave frequency of a user. The term “entrainment” began to be accepted for such devices: “This phenomenon, in which one regular cycle locks into another, is now called entrainment, or mode locking.” (Gleick, Chaos: Making of a New Science 1987, Penguin Books, p. 293). An article entitled “Alpha Brain Waves & Biofeedback Training” in the December 1972 Popular Electronics show a system that uses a person's own EEG signal to modulate a tone generator which, in turn, then drives a speaker heard by the same person. The device allowed a person to “hear” his or her own brain signals in an attempt to voluntarily control the frequency. A similar device that allows a person to “see” his or her own brain waves is shown in an article entitled “Mind Power: Alpha” in the July 1976 Radio-Electronics.
U.S. Pat. No. 5,036,858 to John L. Carter, Harold L. Russell and Len Ochs shows the use of EEG electrodes attached to the head of the user along with an amplifier for determining a current brain wave frequency of a user, which is communicated to a computer processor. A new frequency is generated which is between the current brain wave frequency and a desired brain wave frequency and is within a predetermined range of the current brain wave frequency. This has become known as electroencephalographic entrainment feedback if it is used to “lock” the current brain wave frequency into a desired frequency.
U.S. Pat. No. 5,365,939 to Len Ochs provides a method of “exercising” the brain by using a device producing audio and visual stimulation to move a user's brain wave frequency back and forth between predetermined frequency levels.
In U.S. Pat. No. 6,081,743 to John L. Carter, Harold L. Russell, W. Daniel Vaughn and Robert R. Austin, a method for treating an individual is described by determining a brain wave frequency which corresponds to a highest evoked response of the individual, entraining the brain wave frequency to the brain wave frequency corresponding to the highest evoked response, and then maintaining the brain wave at that frequency for a predetermined length of time. The highest evoked response is described as the highest EEG response or the highest cerebral blood flow (CBF) of the individual or even some other measure.
Two patents in application at the time of the disclosure of the present invention relate to the present invention. In Application No. 20010003799 by Birinder B. Boveja an apparatus and method for adjunct (add-on) therapy utilizing an external stimulator is described to stimulate a cranial nerve according to a predetermined program.
In Application No. 20010007950 by Richard B. North et. al. a neurostimulation system and method is described that includes an implantable stimulator and patient interactive computer. Also, the '950 application requires patient interaction.
Prior methods of neurostimulation for therapeutic purposes have in many ways attempted to ameliorate brain functioning by providing the brain with electrical energy that is designed to be a reflection of the brain's own activity, often with the intent of modifying the brain electricity to follow, or entrain to, a desired frequency, range of frequencies, or relationship among frequencies, or alternately to target theoretically and empirically derived frequency states as a goal of training or therapy. However, little attention has been given to signal design for overcoming the complex and composite impedance presented by the tissues of the head. Such signals, when properly constructed, will limit attenuation of neurostimulation signals for improved effectiveness in patient treatment.
Regarding the concept of conductivity, it is known that the tendency of any conductive material to limit the flow of electrical charge, otherwise known as electrical current, is known as impedance. In general terms, attenuation of current flow is proportional to the magnitude of a material's impedance. The impedance of a substance is a function of its material and physical properties. Environmental factors, such as temperature, also influence the impedance of a material. Most significant to the disclosure of the present invention is the relationship between impedance of a material and the frequency of the electrical signal being conducted through the material.
In its most fundamental terms, three electrical effects govern impedance: resistance, capacitance and inductance. Resistance is a fundamental form of impedance that is constant in time. Therefore, the frequency of a signal has no effect on resistance. However, both capacitive and inductive effects are functions of frequency. Inductive reactance, the formal name for impedance due to inductance, is proportional to frequency. Thus, the higher the frequency of a signal is, the higher will its corresponding inductive reactance be. Capacitive reactance, the formal name for impedance due to capacitive effects, is inversely proportional to frequency. Thus, as a signal's frequency increases, the impedance of a material due to capacitance decreases. For very high frequencies, capacitive reactance can become very small, and the resulting attenuation of the flow of current will be correspondingly less.
The head is comprised of a series of tissues that can be approximately thought of as composite layers surrounding the brain. Specifically, these layers of tissue include the dermal layers of the scalp, the skull, the meninges, the cerebral spinal fluid that bathes the brain, and the brain itself including both healthy tissues and unhealthy tissues such as lesion matter forming in the region of damage. The impedance of the tissues of the head is known to be complex in nature, that is, they have both a resistive component and a capacitive component. Thus, the overall impedance of these tissues will be a function of signal frequency because of capacitive reactance, and an electrical circuit model of these impedances must account for this fact. Because of the nature of this impedance, a higher frequency signal will pass through the tissues with much less attenuation.